Production Practices and Quality Assessment of Food Crops. Vol. 1

Most agricultural chemicals have been developed and rates determined using
high-volume spraying techniques. In low volume spraying, maintenance of the
dosage rate is important to avoid loss of efficacy of the chemical. McArtney and
Hughes (1992) discuss the importance of maintaining a constant product rate per
hectare while reducing water volumes. This means that tank concentrations are
increased which could increase the risk of phytotoxicity through excess deposition
of spray, or failure of the spray through insufficient spray deposited.
With the advent of CDA machines in the 1970’s which were able to produce
droplets within the 60–100 µm range, there was a marked reduction in both spray
drift and the loss associated with large droplets. Several machines had the capability
of conforming closely to this range and utilised spinning discs or rotary
cages to produce a consistent range of droplets. Further, droplet size could be
adjusted by altering flow rates and pressure. This allowed the alteration of spray
application when needed, for instance, under warm spraying conditions in some
countries it is advisable to increase droplet mean diameter to allow for evaporation
effects.
5.3.1. Application of CDA technology
Spray Technology in Perennial Tree Crops 97
Work with CDA machines in Australian orchards in the 1980’s demonstrated that
they were at least as biologically effective at 200 L/ha as an air-blast sprayer at 6000
L/ha (Oakford et al., 1991). Further trials with both Micron and Micronair CDA
machines showed that ultralow volumes as low as 25 L/ha were capable of producing
results similar to high volume applications (Oakford et al., 1994a). However,
spinning discs and rotary atomisers were not robust, and were subject to frequent
breakdowns and damage. This combined with the almost invisible output at ultralow
volumes meant orchardists were reluctant to accept this large technological change
in spray application, hence uptake of ULV spraying in orchards has been slow,
particularly in Australia and New Zealand where tree size tends to be larger than
in the UK.
5.3.2. Application of airshear technology
Airshear technology, which works on the principle of high air velocity (282–370
km/hr) and low liquid pressures (68-170 kPa) has proved to be more popular, at least
in Australia, than CDA technology. These machines are more robust, utilising high
speed turbine fans to produce droplets in the 50–130 µm range. While not producing
as concise a range of droplet sizes as the spinning disc or rotary cage atomisers,
Oakford et al. (1994b, 1995) demonstrated that airshear sprayers were superior to
air-blast sprayers using high pressure hydraulic nozzles. Experiments in Australia
by Oakford et al. (1995) using airshear have shown that at volumes of 200 L/ha
the chemical dosage rate could be reduced by 25%. Oakford et al. (1994b) reported
that reducing the output volume from 200 to 100 L/ha significantly depressed the
effectiveness of the airshear machine, however Oakford et al. (1995) found 100 L/ha
as effective as 200 L/ha. These authors also reported a marked fall off in
performance at 800 L/ha. With higher water volumes, airshear nozzles lose their